compatibility flag

src/awk.gmo

@@ -54,6 +54,12 @@ the index of the first character is 1!
 => https://www.gnu.org/software/gawk/manual/html_node/Bitwise-Functions.html bit-manipulation functions
 => https://www.gnu.org/software/gawk/manual/html_node/index.html gawk user's guide
 
+note: to run in compatibility mode, without gnu extensions, use the -c or --traditional flag:
+
+```
+$ awk -c
+```
+
 # other notes
 
 ## loop through the elements of an array